This invention relates to chemically enhanced ion beam etching, and in particular, to using a focused ion beam to selectively etch inter layer dielectrics deposited during integrated circuit fabrication.
Integrated circuits are fabricated by growing, depositing, diffusing, and etching thin layers of conductors, insulators, and semiconductors onto a substrate of a semiconductor material, such as silicon or gallium arsenide wafer. To keep the fabrication processes operating properly, or to diagnose and correct the process when a defect does occur, process engineers must be able to quickly examine the various processed layers.
A primary tool used for examining, analyzing, and repairing processing layers is a focused ion beam (FIB) system. FIB systems improve manufacturing yields by identifying and analyzing defects on in-process wafers, allowing the source of defects to be located and corrected. For example, layers can be sputter-etched by an FIB system to expose underlying layers for observation and testing, or cross sections can be cut to expose the edges of multiple layers to observe layer thickness, uniformity, and inclusions. FIB systems can also form images of micrcscopic features and can be used to repair or test integrated circuits by depositing conductive or insulating material.
The processing layers exposed by the removal of covering material using FIB etching can be examined either using the imaging capability of the FIB system, or using a scanning electron microscope (SEM). The electron beam of an SEM causes less sample damage than does the ion beam of an FIB system, and the SEM is typically capable of forming a higher resolution image. SEMs are often available within the same vacuum chamber as an FIB system, such as in the DualBeam.TM. family of FIB Systems from FEI Company, the assignee of the present invention. In such a system, a cross section of the processing layers can be milled and then observed within the same vacuum chamber, with little or no movement of the sample. Such a system is particularly well suited to process control applications, where specimens must be analyzed quickly to provide feedback to a production line.
Many of the layers in an integrated circuit are composed of relatively non-conductive materials that are used to separate conductive layers or as passivation and protection Layers for the chip. Such layers are known as interlayer dielectrics (ILDs). ILDs include deposited oxides of various densities, thermal oxides, spun on glass, and nitrides. When ILDs are cross-sectioned with a focused ion beam and viewed, it is often impossible to distinguish among them. Thus, individual layer thickness cannot be determined and process engineers cannot isolate defects to a particular layer.
To distinguish between different ILD layers, it has been necessary to remove the specimen from the vacuum chamber and etch it in a bath of wet chemicals, such as ammonium fluoride (NH.sub.4 F) and hydrofluoric acid (HF), or a combination of NH.sub.4 F, HF, and acetic acid. The wet etching process etches the various layers slightly differently, so that upon rinsing, cleaning, and re-inserting into a vacuum chamber, the different layers can be viewed. Unfortunately, the time required to perform the multitude of steps involved in this process makes it unsuitable for real-time process control. Moreover, the etching of a chemical bath cannot practically be limited to the area of interest; the entire wafer must be etched to increase the contrast in a cross section of a single device of interest.
It has also been found that plasma etching using gases such as CF.sub.4 and C.sub.4 F.sub.8, enhances the contrast between the layers that were exposed by focused ion beam milling. Plasma etching is performed in a plasma chamber associated with a plasma-generating device. As in the wet chemical process described above, it is necessary to remove the specimen from the FIB vacuum chamber, place it in the plasma chamber for etching, and then place it in another high vacuum imaging instrument, such as a scanning electron microscope, for observation. The time required to switch between machines makes the plasma etching process for contrast enhancement unsuitable for production support when process engineers need answers quickly to keep a fabrication line running smoothly.